The present invention relates to a process for continuous casting of a molten metal. The present invention relates to a process for continuous casting of a molten metal comprising repeatedly changing a property, such as an amplitude, a frequency or a phase, of an alternating current to be exerted so that an electromagnetic force is applied to the molten metal to separate it from the mold, whereby the instability of the initial solidification is suppressed while imparting an operation for lubrication between the mold and the molten metal and a surface property improvement is effected.
In continuous casting, powder is generally added to the upper surface of a molten metal pool within a mold. The powder is melted by heat from the molten metal, and the relative movement of the mold wall oscillating up and down and the solidified shell being drawn at a constant rate causes the molten powder to flow into a gap between the wall and the shell. The meniscus and the tip of the solidified shell are deformed by dynamic pressure generated during the inflow of the molten powder. Since the deformation is repeated at the cycle of the mold oscillation, periodic shrinkages, termed oscillation marks, are formed on the cast metal surface.
The formation of regular oscillation marks having a depth of an ordinary magnitude is known to contribute to the stabilization of casting operation and the cast slab surface quality. However, when the oscillation marks are excessively deep, cast metal surface defects may be formed. Moreover, in addition to the problem that the marks themselves are overly deep, there arise problems in, for example, that positive segregation of Ni is formed in the mark bottom portion and surface grinding the cast slab surface is required when austenitic stainless steel is continuously cast, and that an increase in the number of bubbles and inclusions trapped in the marks is observed as the marks are formed even when common steel is cast. In some cases, even the yield of the cast slab is lowered.
On the other hand, rape seed oil was previously used in place of the powder in continuously casting a metal having a small cross section, for example, a billet. In such continuous casting of metal having a small cross section wherein teeming is conducted without using an immersion nozzle, the powder cannot be used because powder is entrained by the teeming flow. It is known that the rape seed oil burns in the meniscus to form graphite, which prevents the solidified shell from sticking the mold wall. However, it is difficult to obtain regularly formed distinct oscillation marks of the surface of the resultant cast slab. The stability of the casting operation and that of the cast metal quality are inferior compared with the stability in casting using the powder.
As a method for controlling the initial solidification as described above, Japanese Unexamined Patent Publication (Kokai) No. 52-32824 has proposed a method for improving the surface properties of cast slab in a process for continuous casting by teeming a molten metal 2 together with a lubricant 4 into a water-cooled mold 1 which oscillates at a constant cycle and continuously drawing downward, the method comprising continuously applying an alternating current to an electromagnetic coil 5 provided around the periphery of the mold as shown in FIG. 2 so that the electromagnetic force generated by the alternating electromagnetic field makes the molten metal 2 form a convex curve. Moreover, Japanese Unexamined Patent Publication (Kokai) No. 64-83348 proposes a method for further improving the surface properties in powder casting at the time of imparting an electromagnetic force to a molten metal within a mold, using an electromagnetic coil, by intermittently applying an electromagnetic force through imparting the alternate magnetic field in a pulse form as shown in FIG. 3.
As disclosed in Japanese Unexamined Patent Publication (Kokai) No. 52-32824, the surface properties of the cast slab have been improved by continuously applying an electromagnetic force to the molten metal within a mold using an electromagnetic coil. However, the applied electromagnetic field not only changes the meniscus configuration but also heats the molten metal which is solidifying within a mold. As a result, the initial solidification does not necessarily progressed stably. Furthermore, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 64-83348, when an electromagnetic force is intermittently applied to a molten metal within a mold within an electromagnetic coil, the inflow of powder between the solidified shell and the mold wall is accelerated, and the surface properties of the cast slab are improved. However, in the case of a rapid on-off pattern as shown in FIG. 3, a wave motion is sometimes generated on the surface of the molten metal pool. The wave motion causes a problem in that it remains during a non-current stage, so that turbulence of the meniscus of the molten metal pool takes place to exert adverse effects on the application of an electromagnetic force and sometimes causes powder trapping in the solidified shell in an extreme case. On the other hand, in a continuous process in which a lubricant such as powder flowing between the solidified shell and the mold as a liquid state from the meniscus is not used, the casting operation and the surface properties of the cast slab must be improved so that they become the same as those after casting using powder.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2-37943 discloses a method for improving the surface properties without using a conventional lubricant such as powder and rape seed oil by initiating solidification at a level lower than the meniscus to exclude the turbulence of the meniscus. In the method, such a refractory material having a predetermined electric conductance as graphite and alumina graphite is used as a mold, and the mold is made to generate heat by an electromagnetic coil provided therearound so that the solidification level of the steel is controlled. Thus, the method makes it possible to continuously cast while the molten metal is solidified under the molten metal surface. According to the patent mentioned above, when a molten metal solidifies on a heated mold wall, a solid-liquid coexisting phase inevitably exists immediately before the portion which becomes a complete solid phase in the drawing direction of the cast slab. Since the solid-liquid coexisting phase portion does not have a sufficient strength, the portion sometimes remains separately at the time of drawing the cast slab. Consequently, it has been impossible to stably conduct casting operation. As described above, the applied electromagnetic force is also exerted on the molten metal to decrease contact pressure between the metal and the mold, namely to decrease contact resistance therebetween. When the force is increased for the purpose of stabilizing initial solidification, the amounts of heat generated in the mold and the metal increase and, as a result, stabilization has not been achieved.
An object of the present invention is to provide a process for continuously casting molten metal which solves the problems caused by the conventional casting process imparting an electromagnetic force, and which restrains the instability of the initial solidification and stably achieves the effects of improving lubrication and the effects of improving the surface properties of the cast metal. A further object of the present invention is to simultaneously provide a process for continuously casting molten metal without using powder which process stabilizes the initial solidification influencing the surface properties of the cast slab with stabilization of the casting operation.
As shown in the schematic view of the principle of the generation of electromagnetic force in FIG. 17, in the process for continuously casting a molten metal in the present invention, an alternating current is applied to a solenoidal electromagnetic coil 5 which is provided so that it surrounds a continuous casting mold or is embedded in the side wall of the mold, and continuous casting is conducted while an electromagnetic force 18 is applied to a molten metal 2 which has been poured into the mold and which immediately starts to solidify. The direction of the electromagnetic force 18 is determined by the direction of an induction current 20 and that of an induction magnetic field 19, and in the present invention the electromagnetic force is always exerted on the molten metal 2 in such a direction that the molten metal 2 is separated from the wall of a mold 1. In the process, the alternating current to be applied is made step-like as shown in FIG. 1 and, as shown in FIG. 1(a), a large current stage is designated as t1 and a small current stage is designated as t2. A large current for applying an electromagnetic force necessary for changing the meniscus configuration is combined with small current having a function different from that which changes the meniscus configuration, before and after the large current. Alternatively, as shown in FIG. 1(b), a large current is applied to impart an electromagnetic force necessary for changing the meniscus configuration, and then a small current is applied to obtain a function different from that which changes the meniscus configuration. A pair of the current applications or a plurality of pairs thereof are conducted, and subsequently a non-current stage (toff) is provided, whereby the instability of the initial solidification of the molten metal generated during continuous current or pulsed current (the application stage being termed ton) is suppressed and the effects of improving lubrication and improving the surface properties of the cast slab are stably obtained. Furthermore, in the above current application, the proportion of the large current application time effecting the meniscus deformation to the current application time within one period is preferably determined to be at least 0.2 and up to 0.8. As a result, the effects of improving lubrication between the mold wall and the solidified shell and improving the surface properties of the cast slab can be maximized.
Furthermore, in a process for continuously casting a molten metal without using the powder or using a substance such as rape seed oil which does not exist as a liquid state in the meniscus of the molten metal, the alternating current is applied to an electromagnetic coil provided so that the coil surrounds a continuous casting mold, and as a result an electromagnetic force is intermittently applied to the meniscus of the molten metal within the mold. Consequently, the periodic deformation and overflow of the metal which is solidifying in the meniscus are accelerated, and regular oscillation marks are obtained. It becomes thus possible to stabilize the initial solidification in continuous casting. That is, the concrete technical features of the process are as described below.
First, an alternating current which is periodically changed in its amplitude, frequency, phase, or the like, namely its waveform is applied to a solenoidal electromagnetic coil provided around the outer periphery of a continuous casting mold which oscillates with a constant period. As a result, an electromagnetic force which changes in accordance with the alternating current is applied to a molten metal poured into the mold.
When the period of applying the electromagnetic force is synchronized with the period of the mold oscillation and the application stage is conformed to the negative strip stage, uniform oscillation marks are formed in the peripheral direction of the cast slab surface. A cast slab having good surface properties can thus be obtained. Moreover, when the stage of applying the electromagnetic force conforms to the positive strip stage, the formation of oscillation marks on the cast slab surface is suppressed, and a cast slab having a smooth surface can be obtained.
Secondly, a periodically changing alternating current is applied to a solenoidal electromagnetic coil provided around the outer periphery of the wall of a continuous casting mold without oscillation, and as a result an electromagnetic force which changes in accordance with the alternating current is applied to a molten metal poured into the mold. Marks equivalent to oscillation marks are thus formed on the cast slab surface.
In the first and the second process, there are three procedures as mentioned below as concrete means for applying a periodically changing alternating current to an electromagnetic coil, whereby an electromagnetic force changing in accordance with the alternating current is applied to a molten metal poured into a mold.
(1) A pulsed alternating current is applied to the electromagnetic coil so that one period of the electromagnetic waveform becomes an intermittent magnetic field formed by an alternating magnetic field application stage and an alternating magnetic field nonapplication stage, whereby an intermittent electromagnetic force is applied to the molten metal poured into the mold.
(2) An alternating current, changing while having strong and weak amplitudes, is applied to the electromagnetic coil so that a nonapplication stage is alternating magnetic field is not present in one period of the resulting electromagnetic waveform, whereby an electromagnetic force which changes in accordance with the amplitude of the alternating current is applied to the molten metal poured into the mold.
(3) An alternating current changing while having high and low frequencies is applied to the electromagnetic coil so that a nonapplication stage of alternating magnetic field is not present in one period of the resulting electromagnetic waveform, whereby an electromagnetic force which changes in accordance with the frequency of the alternating current is applied to the molten metal poured into the mold. Among these procedures, the procedures (2) and (3) are step-like current procedures, which will be described later, and impact step-like electromagnetic waveforms.
In the procedures as mentioned above, the desired stable control becomes possible changing the alternating current applied to the electromagnetic coil in manners as described below, regardless of whether or not the powder is used.
That is, when the mold oscillates, the frequency of the mold oscillation (fm) and the frequency of the alternating current (fp) are set in the range defined by the formula: 0.69xe2x89xa6ln (fp/fm)xe2x89xa69.90. A modulated current is applied to the electromagnetic coil in place of periodically stressing and weakening the amplitude of an alternating current applied to the electromagnetic coil, and the frequency of signal waves of the modulated current is set at the frequency of the mold oscillation. In addition, the frequency of carrier waves (fc) of the modulated current and the frequency of the mold oscillation (fm) are set in a range defined by the formula: 0.69xe2x89xa6ln(fc/fm)xe2x89xa69.90. An amplitude-modulated current, a frequency-modulated current or phase-modulated current is selected as the modulated current. When the mold does not oscillate, a frequency in the range from 1 to 5 Hz, which is usually used for the mold oscillation, is selected as fc.
An apparatus of the present invention relates to continuously casting a molten metal wherein said apparatus is used for a solidifying process under meniscus without using a lubricant,
comprising a vessel for holding a molten metal in a molten state which vessel has a heat insulated structure of a heating function, a water cooled mold for solidifying the molten metal which mold is connected to the vessel, a solenoidal electromagnetic coil surrounding the molten metal provided in the connecting portion of the vessel and the water cooled mold and a power source or waveform generator which applies an alternating current, which periodically changes its amplitude or waveform, to the electromagnetic coil.